{11 s{11 {11 {11 impurity maser

ABSTRACT

A maser is disclosed in which a predominantly optically transparent crystal of either single or polycrystalline characteristics is used, the crystal containing at sites of cubic symmetry ions having a 2S1/2 ground state, an excited state, and a nonzero nuclear magnetic moment, and being mounted in a low temperature device, subjected to a magnetic field, and optically pumped by light energy.

[ 1 July 18,1972

United States Patent Holton [54]: 51 2 IMPURITY MASER OTHER PUBLICATIQNS Rauber et al., Localized S -State Centers in ZnS, l8 Physica Status Solidi 125 132 (1966) QC l76.AlP5

[72] Inventor: William C. Holton, Dallas, Tex.

[73] Assignee: Texas Instruments Incorporated, Dallas,

Tex.

Primary Examiner-Rodney D. Bennett, Jr. Assistant Examiner-N. Moskowitz 22 Filed: June 10, 1968 Attorney-Samuel M. Mims, Jr., James 0. Dixon, Andrew M. Hassell, Harold Levine, James C. Fails and Melvin Sharp [21] Appl. No.:

ABSTRACT A maser is disclosed in which a predominantly optically transparent crystal of either single or polycrystalline characteristics is used, the crystal containing at sites of cubic symmetry ions havinga S ground state, an excited state, and a nonzero nuclear magnetic moment, and being mounted in a low temperature device, subjected to a magnetic field, and optically pumped by light energy.

.0 up, mmw 313 3 .1 m M m a m M M MC S m m u u n A m m m T m m m S R w 2w H w can SHLd Umm mum m own 5 .330/4 Sabisky et al. 22 Claim, 3 Drawing figures DC MAGNETI PATENTEI] JUL 1 8 I972 SHEET 1 [IF 2 lHQFl PATENTED JULI 8 I972 SHEET 2 OF 2 s,,, IMPURITY MASER This invention relates to improvements in masers and more particularly to improvements in optically pumped masers.

Masers, as commonly known in the art, are devices which amplify microwave energy by coherently discharging an inverted population distribution of electrons in phase with the signal desired to be amplified. The inverted populafion is maintained by pumping energy such as microwave or light energy into a maser crystal which contains certain impurity ions.

So that a more complete understanding of the uniqueness of present invention can be had, basic maser theory is herein presented in a somewhat elementary form. The discrete electron energy levels of the ions in the crystal are first split by applying a magnetic field to the maser crystal. These split levels are the so-called Zeeman levels which can be experimentally determined by observing the electromagnetic energy absorption characteristics of the atoms. Between such Zeeman levels, an atomic resonance is seen, that is, at a particular electromagnetic wave frequency, a proportionately greater quantity of energy is absorbed by the atoms than at other frequencres.

The microwave amplification characteristic of masers is the result of establishing and maintaining an inverted population of electrons on these Zeeman levels. An inverted population of electrons is defined in an atom if more electrons exist in an electron energy state higher than some lower energy state, and is achieved by imparting sufficient energy to the lower state electrons to cause them to appear in a higher energy state, from which the electrons will eventually return to the lower state population state by relaxation processes. The effect of such an inverted population is to give the maser crystal negative attenuation at the frequency defined by the energy difference between the inverted population states. Thus, when a microwave signal of that frequency is applied to the maser crystal having such negative attenuation property, the signal is amplified.

Optically pumped microwave masers have been proposed using ions having a 5 ground state and no nuclear spin in a crystal having sites in cubic symmetry. In such maser, the inverse electron population is maintained by optically pumping with polarized light. For further details on this subject, reference is made to Sabisky and Anderson in IEEE Journal of Quantum Electronics, Vol. QE-3, No. 7, July, 1967.

Reference is also made to Rauber and Schneider, Localized S,, -States Centers in ZnS in Phys. Stat. Sol. 182225 (1966). FIG. 1 on page 128 of said latter reference shows the Breit- Rabi diagram for the divalent Ga ion in cubic ZnS crystal. It will be noted that with no magnetic field (H=0) there are two energy levels. However, when an external field is applied to the crystal and ion, a Zeeman interaction occurs which results in the energy states dividing into seven discrete levels to define new hyperfine electron energy states. The two zero-field levels (H=O) are produced by the interaction of the nuclear mag netic moment with the unpaired electron moment.

I have discovered that by optically pumping crystals containing ions having a S ground state and having a nuclear magnetic moment by nonpolarized light, an inverted population of electrons is produced on the Zeeman levels. That is, electrons normally seeking to exist in the lower energy state, such as F=l, m,== l or F=2, m,=2, as shown in FIG. 1 of the above Rauber and Schneider reference are optically excited into a higher energy state, for example, to a p electron state or other higher states. They then tend to return by relaxation processes to the original F=l and F=2 states, but in a higher energy state than their beginning state; for example, m =-l rather than 2 on F=2 or, for further example, m,=-2 on F=2, rather than m,= -l on F=l. If a sufficient energy is applied (for example, by optical pumping) more electrons are maintained in the higher energy state than in a lower state, thus creating the inverse population required for maser action. This discovery of an optically induced inverted population among the hyperfine levels makes a large class of ions usable as was, a class which has never before been considered for this purpose.

When a microwave signal to be amplified is applied to a crystal in which an inverted population exists, the electrons of the inverted population discharge photons in phase with the applied signal and cause it to be amplified.

Accordingly, it is an object of the present invention to provide a maser in which ions having a S ground state and nonzero nuclear spin are used in an active crystal. (By an active crystal in this specification is meant a crystal doped with ions in which an inverted population can be maintained.)

It is a further object to provide a maser which is optically pumped by unpolarized light.

It is a further object to provide a maser with a recovery time on the order of 1 microsecond.

It is yet a further object to provide a maser which may use a polycrystalline crystal material having sites of cubic symmetry, as well as single crystalline material. 7 7

It is still a further object to provide a maser for use at frequencies from about 1 GHz to about 1,000 GHz.

It is a still further object to provide a maser which can operate at higher temperatures than heretofore.

It is yet a further object to provide a maser which does not require large magnetic flux to cause electron level splitting, sufficient magnetic flux being obtained, for example, from a simple hand-sized permanent magnet.

It is a still further object to provide a method for amplifying an electromagnetic signal by causing, upon application of the signal, a coherent discharge of an inverted electron population maintained in an active crystal by optically pumping with nonpolarized light.

Other objects, features and advantages of the invention will become apparent from the following detailed description when read in conjunction with the appended claims and attached drawings in which:

FIG. 1 is a perspective view of the maser crystal and its relationship to its associated microwave equipment.

FIG. 2 is a diagram of the apparatus used in the operation of a maser in accordance with the invention.

FIG. 3 is a cut-away view of the dewar shown in FIG. 2.

In accordance with the invention, a maser is provided in which the active material predominantly is an optically transparent crystal. Examples of such crystals are ThO KCl, ZnS, CaF Cs ZrC1 K PtC1 SnO and M 0. It is not intended, however, to limit the invention to these named crystals as there are other crystals and other materials which are equally suitable. It is to be noted that the active material need not be single crystalline as required by previously known maser techniques, but can be polycrystalline. The only limitations on crystals of polycrystalline material, when used, are (1) that it be predominantly optically transparent, and (2) that the impurity site be of cubic symmetry. To be understood is that the crystal may be of single crystalline material, but the invention is not limited in this respect.

A quantity of impurity ions are introduced into the active crystal material. The impurity ions used, however, are limited to those in the class of atmons characterized by an electronic ground state of 8 having a nonzero nuclear magnetic moment so that the hyperfine splitting is large compared to the Zeeman splitting of the energy bands. A ground state of 8 means that the ion has as its lowest energy state a spherical cloud of electron charge having two energy levels. Examples of such ions are Na", Cu", Ag", Au, Ca, Sr*, Ba*, Ga, In, Tl, Si, Sn Pb, and Bi. Again, this listing of useful ions is not intended to limit the invention. Other ions with the named characteristics will work equally well.

Reference is made first to FIG. 1 which is a perspective drawing showing a maser crystal in accordance with the present invention and its relationship with its associated microwave equipment. The maser crystal 10 is predominantly of optically transparent material into which ions having a S ground state, a nonzero nuclear magnetic moment, and optical absorption bands have been introduced. (The crystal may itself have optical absorption bands which play a role in achieving the inverted population.) Maser crystal is mounted upon a slow-wave of meander" strip 1 l, which can be of any suitable material capable of conducting the signal, and configurated as shown, the crystal 10 being largely cut away except at its ends to illustrate the character of the meander configuration. It is to be understood that the use of the meander strip is only one of many ways by which microwave energy can be introduced into and removed from the maser crystal. The microwave signal to be amplified is directed down wave guide 12 and introduced into the meander strip 11 at feed point 13. The wave then progresses along meander strip 11, and causes the inverted population to coherently discharge along the length of maser crystal 10 to thereby amplify the wave. The amplified wave then is removed into wave guide 14 through a feed point, not shown but similar to feed point 13.

With reference to FIGS. 1, 2 and 3, there is shown in FIG. 2 a diagram of the apparatus used in the operation of the maser, and in FIG. 3 a cut-away view of the dewar of FIG. 2.

The maser crystal 10 must be placed under the influence of a d-c magnetic field. The crystal 10, meander strip 11 and waveguide of FIG. 1 are therefore mounted in a dewar between the poles of a magnet 21. The direction of the magnetic field, H, as shown in FIG. 1, is across the maser crystal.

The dewar 20 is maintained at a low temperature, perhaps 12 K or lower. This is accomplished, for example, by causing liquid helium or liquid hydrogen in the dewar to absorb the heat energy of the maser crystal. The means for operating a dewar at such low temperatures are so well known in the art that they are not discussed here nor shown in the drawings.

The maser crystal 10, under the influence of a magnetic field and at a low temperature, as above described, is then pumped with light energy. Because of the hyperfine interaction characteristics of the ions of the 8 group having nonzero nuclear magnetic moment, it is unnecessary to pump the maser crystal 10 with light of a particular frequency; however, it is to be understood that to optimize the efficiency of the pumping it may be of advantage to use light which is limited to particular frequencies. This is of special advantage to the invention as ordinary light sources can be used, such as, for ex ample, incandescent, arc, of mercury vapor lamps, or perhaps by lasers. To achieve the optical pumping, light is directed from light source 22 (see FIGS. 2 and 3) through a focusing lens 23, a filter 24, a window 25 in the metallic dewar 20, and a distributing lens 26 above the crystal. The type of light source and manner of directing the light energy to the crystal is immaterial, and the naming of particular type sources above is for example only as it is not intended to be a limitation on the invention. It is also to be noted that filter 24 is provided for the purpose of filtering infrared rays to prevent the light energy from increasing the temperature of the maser crystal in the dewar.

To prevent discharge of the inverted population by extraneous signals, the maser crystal 10 is placed in a shielding box 28 (shown in FIG. 2), which also provides a convenient mount for distributing lens 26.

The frequency range in which the maser operates is determined by the crystal material, the ions used, and the magnetic field strength. The frequency may bear an inverse relationship with the magnetic field strength. For example, I have experimentally determined that the frequency of Ga ions in a ZnS crystal with an applied magnetic field strength of 10,000 Gauss is about 10 GHz.

Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.

What is claimed is:

1. In a maser device for causing an inverted electron population in a crystal which discharges upon selective application of a signal, the improvement comprising:

an active crystal of material having sites of cubic symmetry which are selectively doped with impurity ions having an electron ground state of 8, an excited state, and a nonzero nuclear magnetic moment.

2. The maser device of claim 1 wherein said crystal is zinc sulfide (ZnS) and said impurity ions are gallium ions (Ga*).

3. The maser device of claim I wherein said crystal is zinc sulfide (ZnS) and said impurity ions are thallium ions (11).

4. The maser device of claim 1, wherein said ions have a hyperfine splitting produced by the interaction of the nuclear magnetic moment with the electron magnetic moment which is larger than the Zeeman splitting of the levels.

5. The maser device of claim 1, wherein said crystal has optical absorption bands.

6. The maser device of claim 1 wherein said crystal is of optically transparent material and said ions have optical absorption bands.

7. An optically pumped maser comprising:

a. a crystal of material having sites of cubic symmetry at which are selectively doped impurity ions having an electron ground state of 5 an excited state, and a nonzero nuclear magnetic moment,

b. a light source disposed to cause light to impinge upon said crystal,

c. a d-c magnetic field traversing said crystal,

d. means to introduce into and extract from said crystal a microwave signal, and

e. means for maintaining said crystal at a temperature below 12 Kelvin,

whereby light from said source of light causes an inverted electron population to exist in said crystal which coherently discharges upon application of said signal thereby amplifying said signal.

8. The maser of claim 7 wherein said crystal is zinc sulfide (ZnS) and said ions are thallium ions (11).

9. The maser of claim 7 wherein said crystal is zinc sulfide (ZnS) and said impurity ions are gallium ions (Ga'*).

10. The maser of claim 7 wherein said ions have a hyperfine splitting produced by the interaction of the nuclear magnetic moment with the electron magnetic moment which is larger than the Zeeman splitting levels.

11. The maser of claim 7 wherein said crystal has optical absorption bands.

12. The maser of claim 7 wherein said crystal is of optically transparent material and said ions have optical absorption bands.

13. An optically pumped maser, comprising:

a. a crystal of material having sites of cubic symmetry at which are selectively doped impurity ions having an electron ground state of 8 an excited state, and a nonzero nuclear magnetic moment,

a conductive meander strip beneath said crystal,

0. a pair of wave guides electrically connected to said meander strip,

. means for maintaining said crystal at a temperature below 12 Kelvin,

e. means for producing a d-c magnetic field across said crystal, and

f. light from a source impinging upon said crystal for optically pumping said crystal, thereby to produce an inverted electron population therein,

whereby upon a microwave signal entering one of said pair of wave guides causes said inverted electron population to coherently discharge and thereby amplify said signal, said amplified signal emerging from the other of said pair of wave guides.

14. The maser of claim 13 wherein said crystal is of optically transparent material and said ions have optical absorption bands.

15. The maser of claim 13 wherein said crystal has optical absorption bands.

lOl045 0721 16. In the process of amplifying an electromagnetic signal by the coherent discharge of an inverted electron population maintained within a crystal by optically pumping with nonpolarized light, the step of:

introducing the electromagnetic signal into the crystal of material having sites of cubic symmetry at which are selectively doped impurity ions having a 8 ground state, an excited state, and a nonzero nuclear magnetic moment thereby to coherently discharge the inverted electron population and amplify the signal.

17. The step of claim 16 wherein said crystal has optical absorption bands.

18. The step of claim 16 wherein said crystal is of optically transparent material and said ions have optical absorption bands.

19. The method of amplifying an electromagnetic signal comprising the steps of:

a. irradiating with unpolarized light an active crystal of 7 material having sites of cubic symmetry/at which are selectively doped impurity ions having a S ground state, an excited state, and a nonzero nuclear magnetic moment, thereby to create an inverted electron population in the crystal,

b. introducing the electromagnetic signal into the crystal, thereby coherently discharging the inverted electron population to amplify the signal, and

c. removing the amplified signal from the crystal.

20. The method of claim 19 wherein said active crystal has optical absorption bands.

21. The method of claim 19 wherein said crystal is of optically transparent material and said ions have optical absorption bands.

22. In a maser device comprising crystal means having an inverted electron population which coherently discharges upon application of a preselected signal, and means for introducing into said crystal means a microwave signal and for extracting from said crystal means an amplified microwave signal, the improvement wherein said crystal means comprises an active crystal of material having sites of cubic symmetry at which are selectively doped impurity ions having an electron ground state of 5 an excited state, and a nonzero nuclear magnetic moment. 

2. The maser device of claim 1 wherein said crystal is zinc sulfide (ZnS) and said impurity ions are gallium ions (Ga2 ).
 3. The maser device of claim 1 wherein said crystal is zinc sulfide (ZnS) and said impurity ions are thallium ions (TI2 ).
 4. The maser device of claim 1, wherein said ions have a hyperfine splitting produced by the interaction of the nuclear magnetic moment with the electron magnetic moment which is larger than the Zeeman splitting of the levels.
 5. The maser device of claim 1, wherein said crystal has optical absorption bands.
 6. The maser device of claim 1 wherein said crystal is of optically transparent material and said ions have optical absorption bands.
 7. An optically pumped maser comprising: a. a crystal of material having sites of cubic symmetry at which are selectively doped impurity ions having an electron ground state of 2S1/2, an excited state, and a nonzero nuclear magnetic moment, b. a light source disposed to cause light to impinge upon said crystal, c. a d-c magnetic field traversing said crystal, d. means to introduce into and extraCt from said crystal a microwave signal, and e. means for maintaining said crystal at a temperature below 12* Kelvin, whereby light from said source of light causes an inverted electron population to exist in said crystal which coherently discharges upon application of said signal thereby amplifying said signal.
 8. The maser of claim 7 wherein said crystal is zinc sulfide (ZnS) and said ions are thallium ions (TI2).
 9. The maser of claim 7 wherein said crystal is zinc sulfide (ZnS) and said impurity ions are gallium ions (Ga2 ).
 10. The maser of claim 7 wherein said ions have a hyperfine splitting produced by the interaction of the nuclear magnetic moment with the electron magnetic moment which is larger than the Zeeman splitting levels.
 11. The maser of claim 7 wherein said crystal has optical absorption bands.
 12. The maser of claim 7 wherein said crystal is of optically transparent material and said ions have optical absorption bands.
 13. An optically pumped maser, comprising: a. a crystal of material having sites of cubic symmetry at which are selectively doped impurity ions having an electron ground state of 2S1/2, an excited state, and a nonzero nuclear magnetic moment, b. a conductive meander strip beneath said crystal, c. a pair of wave guides electrically connected to said meander strip, d. means for maintaining said crystal at a temperature below 12* Kelvin, e. means for producing a d-c magnetic field across said crystal, and f. light from a source impinging upon said crystal for optically pumping said crystal, thereby to produce an inverted electron population therein, whereby upon a microwave signal entering one of said pair of wave guides causes said inverted electron population to coherently discharge and thereby amplify said signal, said amplified signal emerging from the other of said pair of wave guides.
 14. The maser of claim 13 wherein said crystal is of optically transparent material and said ions have optical absorption bands.
 15. The maser of claim 13 wherein said crystal has optical absorption bands.
 16. In the process of amplifying an electromagnetic signal by the coherent discharge of an inverted electron population maintained within a crystal by optically pumping with nonpolarized light, the step of: introducing the electromagnetic signal into the crystal of material having sites of cubic symmetry at which are selectively doped impurity ions having a 2S1/2 ground state, an excited state, and a nonzero nuclear magnetic moment thereby to coherently discharge the inverted electron population and amplify the signal.
 17. The step of claim 16 wherein said crystal has optical absorption bands.
 18. The step of claim 16 wherein said crystal is of optically transparent material and said ions have optical absorption bands.
 19. The method of amplifying an electromagnetic signal comprising the steps of: a. irradiating with unpolarized light an active crystal of material having sites of cubic symmetry at which are selectively doped impurity ions having a 2S1/2 ground state, an excited state, and a nonzero nuclear magnetic moment, thereby to create an inverted electron population in the crystal, b. introducing the electromagnetic signal into the crystal, thereby coherently discharging the inverted electron population to amplify the signal, and c. removing the amplified signal from the crystal.
 20. The method of claim 19 wherein said active crystal has optical absorption bands.
 21. The method of claim 19 wherein said crystal is of optically transparent material and said ions have optical absorption bands.
 22. In a maser device comprising crystal means having an inverted electron population which coherently discharges upon application of a preselected signal, and means for introducing into said crystal means a microwave signal and for extracting from said cryStal means an amplified microwave signal, the improvement wherein said crystal means comprises an active crystal of material having sites of cubic symmetry at which are selectively doped impurity ions having an electron ground state of 2S1/2 an excited state, and a nonzero nuclear magnetic moment. 